Possible blocked duct

Hi ladies since my son was born 11 months ago I've had a small sore lump near my armpit where all the glands are. I am going to get it checked by a Dr but I asked a nurse who said it's unlikely to be anything sinister but it could be a blocked duct.

I never breast fed but this sore lump can sometimes trigger a sensation down Into my breast and that whole breast can feel more sensitive. Hormonal changes seem to make it more obvious. Could this be a duct with dry milk inside? It's been months since I got my supply and it dried up. Nothing on Google says it can happen to formula feeders. Any ideas?